The present invention relates to, a resin component, a resin component for molding and a method for producing it, and more particularly to a resin component for molding including a biodegradable resin and a method for producing it.
In the field of a resin component and a resin component for molding, a biodegradable resin decomposed in a natural atmosphere and the moldings thereof have been hitherto required in view of natural atmospheric protection. The biodegradable resin such as an aliphatic polyester has been especially vigorously studied. Specially, polylactic acid ordinarily has a high melting point (170 to 180° C.) and the moldings made of the polylactic acid are ordinarily transparent and they have commenced to be put into practice depending on their uses. Generally, the moldings made of the polylactic acid are poor in their heat resistance and have glass transition temperature (Tg) of about 60° C. Accordingly, the moldings made of the polylactic acid are disadvantageously deformed when the temperature exceeds the above-described temperature. In the use of the casing of an electric product or a structural material, a heat resistance to the temperature of about 80° C. is required. Therefore, to utilize the moldings for uses requiring the heat resistance, various kinds of investigations have been carried out. The heat resistance referred to in this specification means a sufficiently high rigidity (modulus of elasticity) at about 80° C. as high as 100 MPa.
To increase the heat resistance of biodegradable polyester, for instance, the addition of inorganic filler has been studied. As the inorganic filler, talc or mica or the like having the heat resistance has been studied. For the purpose of improving mechanical characteristics and hardening a resin, a hard inorganic filler having the heat resistance is added to the resin, like what is called reinforcing steel embedded in concrete. However, the mechanical characteristics are imperfectly improved only by adding the inorganic filler to the resin.
The polylactic acid as a typical example of the biodegradable polyester is a polymer capable of having a crystal structure. However, since the moldings of the polylactic acid is ordinarily amorphous, the moldings are apt to be thermally deformed. Thus, a proposal has been provided that the polylactic acid is crystallized and hardened under a heat treatment, for instance, during molding or after molding to improve the heat resistance. When the polylactic acid is crystallized in such a method, it takes extremely much time to crystallize the polylactic acid. For example, a molding cycle of about one minute is ordinarily required in an injection molding. However, it unrealistically takes too much time to completely crystallize the moldings of the polylactic acid in a die. When the polylactic acid is crystallized in such a method, the size of a crystal is of the order of micron to of the order of sub mm. Thus, the crystals themselves of the polylactic acid inconveniently cause a factor of light scattering to become thick in white, so that a transparency is lost. In order to solve these problems, that is, to accelerate the crystallization, the addition of, what is called a nucleus agent begins to be studied.
The nucleus agent forms a primary crystal nucleus of a crystalline polymer to accelerate the growth of the crystal of the crystalline polymer. Further, in a broad sense, the nucleus agent may be regarded as a material for accelerating the crystallization of the crystalline polymer. That is, a material for accelerating the crystallization speed of the polymer may be regarded as a nucleus agent. When the nucleus agent such as the former is added to a resin, the crystals of the polymer become fine, so that the rigidity of the resin is improved or the transparency is improved. Otherwise, when the polymer is crystallized during molding, the entire speed (time) of the crystallization is accelerated. Thus, a molding cycle can be advantageously shortened.
The above-described effects can be exhibited in other crystalline resins as examples. For example, in polypropylene (abbreviate it as PP, hereinafter), the nucleus agent is added thereto, so that the rigidity or the transparency thereof is improved. Nowadays, the PP whose materiality is improved has been put into practical use in many moldings. The nucleus agent includes, for instance, a sorbitol material and its operational function is not completely clarified. However, a three-dimensional network formed by this material is considered to effectively operate. Further, a nucleus agent of metal salt type is also put into practical use for the PP. As such a nucleus agent, for instance, hydroxy-di (t-butyl benzoate) aluminum, sodium bis(4-t-butylphenyl) phosphate, or sodium methylene bis(2,4-di-t-butylphenyl) phosphate, etc. may be exemplified.
However, few effective nucleus agents have been found for aliphatic polyester such as polylactic acid. The above-described talc functions also as a nucleus agent. Talc may seem to be used as an effective nucleus agent depending on its use. In case talc is used as the nucleus agent, when an amount of addition thereof is not several ten %, a satisfactory effect cannot be obtained. Furthermore, when the amount of addition of talc is increased, the component of the resin inconveniently becomes brittle. In such an amount of addition, the component of the resin becomes white and the transparency thereof cannot be absolutely expected.
Even in the aliphatic polyester, the nucleus agent for accelerating the crystallization has been studied so far. For example, as the nucleus agent for accelerating the crystallization, the use of sorbitol material is disclosed in Japanese Patent Application Laid-Open No. hei 10-158369. This publication discloses that the sorbitol material has satisfactory results as a nucleus agent for crystallization in the PP and the sorbitol material is added to polylactic acid to effectively act thereon. In addition thereto, methods for adding a nucleus agent to polyester to accelerate the crystallization thereof are disclosed in Japanese Patent Application Laid-Open No. hei 9-278991, Japanese Patent Application laid-Open No. hei 11-5849 and Japanese patent Application Laid-Open No. hei 11-116783.
However, any of the techniques has not been put into practical use.